Such a wavelength converter is shown, for example, in an article by D. Mahgerefteh et al, "All-Optical 1.5 .mu.m to 1.3 .mu.m Wavelength Conversion in a Walk-Off Compensating Nonlinear Optical Loop Mirror", IEEE Photonics Technology Letters, Vol. 7, No. 5, May 1995, pages 497 to 499. This wavelength converter, which is shown in FIG. 2, has as its central part a nonlinear optical Sagnac interferometer (nonlinear optical loop mirror, NOLM). A fiber composed of a single-mode fiber (3.58 km SM Fiber) and a dispersion-shifted fiber (2.6 km DS Fiber) is formed into a ring with the aid of a 2.times.2 coupler, i.e., a coupler with four ports. At one of the two ports not used for this purpose, light emitted by a laser (1.3 .mu.m Clock) is coupled into the NOLM, which propagates in the NOLM clockwise and counter-clockwise. Connected to the other of these two ports of the coupler is a photodetector. Through a further coupler (WDM), signal light (1.5 .mu.m Data) is coupled into the NOLM in such a way as to propagate in the NOLM clockwise. In the absence of signal light (1.5 .mu.m Data), the light components propagating in opposite directions (1.3 .mu.m Clock) are subject to the same propagation conditions. In the coupler, the two light components (1.3 .mu.m Clock) interfere constructively, and they exit at the port where the light (1.3 .mu.m Clock) is injected. The signal light (1.5 .mu.m Data) may unbalance the NOLM; then, a portion of the light (1.3 .mu.m Clock) will exit at the port of the coupler where no light (1.3 .mu.m Clock) is injected. The signal light (1.5 .mu.m Data) thus determines when light (1.3 .mu.m Clock) exits at this port. The photodetector detects the light (1.3 .mu.m Clock) carrying the information of the signal light (1.5 .mu.m Data).
From H. Bulow et al, "System Performance of a Nonlinear Optical Loop Mirror Used as Demultiplexer for Bitrates of 40 Gbit/s and Beyond", Proceedings SPIE, Vol. 2449, 1995, pages 158 to 167, use of a NOLM as a demultiplexer is known, the demultiplexer being fed with an RZ (return-to-zero) data signal. Also known from this publication is a measure which indicates how well a 1 bit and a 0 bit at an output can be distinguished from one another; this measure is defined as a ratio of the powers of a 1 bit and a 0 bit (extinction ratio, ER). This ratio ER follows from a parameter describing this NOLM, namely the transmission T, which is a function of a phase difference .DELTA..phi.. If only signal light propagates in the NOLM, the transmission is zero. In FIG. 5 (Bulow), the transmission T is shown as a function of the phase difference .DELTA..phi..
The ratio ER should have as high a value as possible, e.g., ER&gt;10 dB. In addition, such a high value should be reached with as little optical input power as possible. Theoretical considerations (Bulow) and measurements on known wavelength converters using a NOLM have shown that for NRZ (nonreturn-to-zero) signals, the ratio ER.congruent.dB and is thus too small to obtain a usable output signal and achieve wavelength conversion.